I don't know if this is as intended, but I noticed that the level 1 field arrows have an extra pixel on them that the level 2, 3, or 4 arrows don't. It sort of looks like it might be there accidentally since it makes the image asymmetric.
I've circled the pixel in the screenshot below. You can see that it is present for all three types of level 1 fields.
I'm going to guess it was a mistake, but, honestly, why would the average user notice. Why were you even looking close enough?
I was using the screen magnifier to zoom in and check each element in an intricate field. When the fields don't have good symmetry or repetition it is harder to visually verify that everything's in line.